Koh Samui weather by month: rain, heat and what the seasons mean for owners

Koh Samui weather by month changes mostly in one way: it is warm all year, and rain is what differs. According to the Thai Meteorological Department (TMD), at its Koh Samui station October, November and December are the wettest months, with November the wettest of all, and February is the driest. Average daily highs move only between 29.0 °C in January and 32.9 °C in May.

This guide reads one official table: TMD’s climate averages for 1991 to 2020 at its Koh Samui station. We read it on 24 October 2026. The report includes stations that observed for at least 10 years within that period, and the page does not say how many years Koh Samui contributes. Travel sites quote different figures for the same month, so we say exactly which station and which period ours covers. An average is not a forecast. Individual years differ, and TMD publishes current forecasts and warnings.

At a glance

• Wettest: October to December. November averages 444.6 mm of rain on 18.7 rainy days, October 294.9 mm on 19.3 rainy days and December 263.1 mm on 14.2. By our sum, these three months bring about half of the year’s 1,994.3 mm.
• Driest: February, with 64.3 mm on 5.6 rainy days. March and April also have few rainy days. January is not dry: 126.8 mm on 11.1 rainy days.
• May to September: 102.2 to 133.6 mm a month, spread over 13 to 15 rainy days.
• Heat: average highs of 29.0 to 32.9 °C and average lows of 24.0 to 26.0 °C. May is the warmest month. Relative humidity averages 78 to 86%.
• For owners: plan work in the apartment, handovers and guest arrivals around the wet months. At The Astra Samui, according to the developer, completion is planned for Q4 2028, which falls in those months.

Koh Samui climate in brief

The Land Development Department (LDD) describes the Thai climate, from the meteorological point of view, as three seasons. The rainy season, with the south-west monsoon, runs from mid May to mid October. The winter season, with the north-east monsoon, runs from mid October to mid February. The summer season, the transition between the two monsoons, runs from mid February to mid May.

There is one exception that matters here. On the east coast of southern Thailand, the LDD says, abundant rain lasts until the end of the year, and November is the wettest month. Koh Samui is on that coast: TMD lists Samui (Surat Thani) among its forecast locations for the southern east coast. So the heaviest rain here comes after the national rainy season has ended. TMD’s outlook for September to November 2026, issued on 20 August 2026, explained the expected November rain in the south by the north-east monsoon and a low-pressure trough over southern Thailand and the Gulf of Thailand.

The wind shows the same switch. At the station, the prevailing wind is from the west from May to October, and from the east, north-east or south-east from November to April.

The table below comes from the TMD station called Ko Samui (WMO index 48550), 4 m above sea level, listed at 09°28′N 100°03′E. Four limits apply:

• One station cannot show how rain differs between coasts, hills or neighbourhoods. We found no official figures by area, so we give none.
• The table shows no sunshine observations for this station, so we give no sunshine hours.
• The table covers 1991 to 2020, so it does not show recent years.
• TMD includes stations with at least 10 years of observations in that period, and the page does not say how many years Ko Samui has.

Month by month

The figures are monthly averages for 1991 to 2020. TMD counts a rainy day as a day with 0.1 mm or more of rain, so a short shower counts as a rainy day.

Source: TMD, Climatological Data of Thailand for the 30-year period 1991–2020, Ko Samui station, page 71. Average high and low are TMD’s monthly mean maximum and minimum temperatures. Humidity is mean relative humidity.

January to April

January follows the wet months and is not dry: 126.8 mm on 11.1 rainy days. February is the driest month. March has only 5.9 rainy days, yet 115.8 mm. That is about 20 mm for each rainy day, against about 11 mm in February and 10 mm in April (our division). The table lists a daily maximum of 414.7 mm for March. By our arithmetic, a day like that adds about 14 mm to an average over 30 years (414.7 divided by 30), and more if fewer years are counted. So judge the dry months by rainy days as well as by millimetres. April is back to 82.9 mm, and average highs reach 31.8 °C.

May to September

May is the warmest month on average, with highs of 32.9 °C, and it has the most thunderstorm days of the year, 11.6. TMD counts a thunderstorm day when thunder is heard at the station. From May to September, rain is 102.2 to 133.6 mm a month on 13 to 15 rainy days. That is more rainy days than January to April but much less rain than October to December.

October to December

Rain climbs sharply: 294.9 mm in October, 444.6 mm in November and 263.1 mm in December. October has the most rainy days of the year, 19.3, and November has the most rain. Humidity peaks in these months, at 85 to 86% in October and November. Average highs fall to between 29.3 and 30.6 °C.

Temperature changes little through the year. The mean temperature runs from 26.8 °C in December to 29.2 °C in May. Over 1991 to 2020, TMD’s table records an extreme high of 38.0 °C, in April and July, and an extreme low of 17.8 °C, in February.

The rainy season

The Koh Samui rainy season is not the one you read about for the rest of Thailand. In TMD’s table the wettest months are October, November and December. The LDD puts the national rainy season at mid May to mid October, and says that abundant rain on the southern east coast lasts until the end of the year. In this guide, the wet months are October to December. Three points from the table help when you plan a visit or a handover.

• It is the heaviest rain of the year. October, November and December together bring 1,002.6 mm, about half of the annual 1,994.3 mm. These are our sums from TMD’s monthly figures.
• Rain falls on most days, but not necessarily all day. October has 19.3 rainy days and November 18.7. A rainy day needs only 0.1 mm, and the table gives no hours of rain, so we cannot say how long it rains on a typical wet day.
• Single days can be very heavy, and not only in these months. The table lists daily maxima of 363.9 mm in November, 259.6 mm in October and 186.0 mm in December. For March it lists 414.7 mm.

If you read that the dry season starts in December, check it against the table. December still has 263.1 mm and 14.2 rainy days. Rain eases in January and is lowest in February. May to September has more rainy days than January to April but far less rain than October to December.

Storms can also disrupt travel. TMD’s outlook for September to November 2026, issued on 20 August 2026, said that some tropical cyclones had a high chance of passing near southern Thailand in October and November, with heavy to very heavy rain in places near their path. It asked the public to follow TMD’s forecasts and cyclone warnings. Do the same if you are on the island then.

After floods in other southern provinces in late 2025, the Tourism Authority of Thailand (TAT) update of 8 December 2025 listed Surat Thani, including Koh Samui, as fully accessible. The same update said that marine transport may adjust schedules because of weather, and it advised travellers to reconfirm ferry times and to check flight status. Lomprayah, a ferry operator, says in its terms that its timetable and routes may change without prior notice, and that this may result from weather. Check flights and ferries before you travel, and check them again the week before.

What the seasons mean for owners

TMD’s table is weather data. This section is our reading of it for an owner. It is not data about rents, prices or bookings, and nothing here is a forecast of rental income.

Letting

We found no official monthly occupancy or booking figures for Koh Samui, so we give none. Guests arrive by air and ferry, and both can change in bad weather. If you plan to let the apartment, ask a rental manager for the monthly booking history of a comparable apartment before you rely on any month.

Work in the apartment

February to April have the fewest rainy days in the table, at 5.6, 5.9 and 8.1, so they suit painting, balcony work and large deliveries. From October to December rain may delay outdoor work, so book tradespeople early.

An empty apartment

Humidity peaks in October and November. If you are away, ask your manager or the building how an empty apartment is checked and ventilated. Our guide to owning a holiday home in Koh Samui covers care while you are away. How much you cool the apartment also drives your electricity bill, which we cover in our guide to the cost of living on Koh Samui.

Handover

The Astra Samui is a low-rise, resort-style condominium in Bophut. According to the developer, completion is planned for Q4 2028. That quarter is October to December, the three wettest months in TMD’s table, so allow time for the inspection and for furniture deliveries, and treat the date as a target, not a guarantee. A wet day can also show leaks or poor drainage that a dry day hides. Our how to buy guide explains the steps from reservation to transfer.

When to plan a viewing trip

The right month depends on what you want to see.

• February to April: the fewest rainy days, 5.6 to 8.1 a month. Average highs rise from 29.4 to 31.8 °C, and the heat builds towards May.
• May to September: moderate rain, 102.2 to 133.6 mm a month, on 13 to 15 rainy days. The wind comes from the west.
• October to December: the wet months. If you plan to live on the island all year, we suggest seeing it once in this season and comparing it with a dry month.
• January: 126.8 mm on 11.1 rainy days, a transition after the wet months.

Whichever month you choose, keep flights and ferries flexible and check TMD’s forecast the week before you go.

We at Varsovia Estate, the sales agent for The Astra Samui, have an office in Maenam on Koh Samui. To plan a viewing, book a call with our team. You can see where the project sits on the location page. For everyday life, read about living in Bophut and compare the areas of the island. To see how the weather on Koh Samui compares with Phuket, read Koh Samui or Phuket.